With the advent of commercial cellular mobile radio, it is now desirable for companies in the communications business to offer information age services, such as remote financial services, and remote data retrieval, to mobile wireless terminals.
Mobile cellular radio systems operate over noisy Rayleigh fading channels. Such mobile cellular radio units often encounter difficulty in recovering data transmitted at the 1200 bit/sec. rate characteristic of the land based telephone network. This difficulty arises because noise including Rayleigh fading is severe.
Rayleigh fading is typically caused by the reflectivity of various types of signal scatterers both stationary and moving. Such signal scatterers, including both natural and man made topological features, give rise to multipath signal propagation. Rayleigh fading is the result of the addition and cancellation of waves propagating along different paths.
Propagation between a base station and a mobile unit is most susceptible to the effects of multipath Rayleigh fading phenomena because all communication is essentially at ground level where there are large numbers of man made and naturally occurring signal scatterers. Rayleigh fading occurs at intervals as low as one to two feet for a vehicle moving in the 25-35 mile per hour range and receiving a cellular radio telephone message over a channel in the 800-900 Megahertz frequency range. In contrast to ground level cellular mobile radio systems, the effects of multipath phenomena are not significant in air-to-ground and satellite-to-earth station communications because the angle of propagation precludes the multipath effects caused by surrounding natural land features and man made structures.
The Rayleigh fading RF channel presents formidable data transmission problems. Consider for example a carrier with a 10 dB mean carrier to noise ratio. For a mobile receiver unit moving at about 35 miles per hour, Rayleigh fading causes the carrier to noise ratio to fade to a level of 10 dB below the mean level approximately 35 times per second. The duration of each such fade averages about 3 milliseconds. When the carrier is modulated with a 1200 bit/sec. stream of data, the duration of each bit is about 0.08 ms. Thus the duration of the typical Rayleigh fade is much longer than the duration of an individual bit. Accordingly, during each fade entire sequences of bits may be destroyed.
If there are 35 Rayleigh fades per second and each fade has a duration of 3 ms then the carrier to noise ratio will be at a level of 10 dB or more below its mean level for about 10% of the time. While the carrier is in the noise, the bit error probability is one half. Therefore, when 1200 bits/sec. are transmitted on a carrier having a mean carrier to noise ratio of about 10 dB, a bit error probability of 5% can be anticipated solely because of Rayleigh fading. For a comparable channel of the land based telephone network, 1200 bits of data can be transmitted per second with a bit error probability that is less than one in ten thousand.
Looking at Rayleigh fading from another point of view, there is an approximately 3-5% probability that a mobile unit moving at 35 mph will receive 4 errors in a given 32 bit word when the mean carrier to noise ratio is about 10 to 15 dB and the data is transmitted at 1200 bits/sec. The probability of such multibit errors per word increases with decreasing vehicle speed.
In order to provide communications age services over the mobile network, the bit error probability of the Rayleigh fading channel must be brought down so that it is comparable or at least closer to the bit error rate of the land based telephone network. One possible approach to this problem is to provide each mobile unit with sufficient computer capacity to execute error correction codes. However, such error correction codes are slow, require a large amount of computer capacity and become vastly more complex as the number of errors per word increases. It is simply not practical to place the mini computers required to execute such error correcticn codes in a mobile unit. Accordingly, error correction codes are not suitable for improving the reliability of data transmitted over Rayleigh fading channels.
Instead, data must be transmitted over Rayleigh fading channels in a manner such that fewer errors occur. Accordingly, it is the object of the present invention to provide a method and an apparatus for transmitting data over a Rayleigh fading channel which results in a significantly reduced bit error probability.